Infrared absorbing green glass

ABSTRACT

A green optical filter phosphate-based glass having a high near infrared, absorption at about 650 nm to 950 nm and a transmission peak transmission at 550 nm to 560 nm and with the steepest slope of the absorption curve occurring between about 570 nm to 650 nm. The glass composition comprising, in mol %: 50 to 60% P 2  O 5 , O to 5% SiO 2 , 20 to 33% Li 2  O, 0 to 25% Na 2  O, 1 to 25% K 2  O, 0 to 5% for each of CaO, Bao, SrO and MgO, 7 to 10% Al 2  O 3 , 3 to 6% CuO, 0.5 to 1.6% Cr 2  O 3 , 0.5 to 2.0% Ho 2  O 3 , 0.5 to 2.0 Dy 2  O 3  and 0 to 2.0% Er 2  O 3 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a green optical filter phosphate-basedglass having a strong near infrared absorption and a high transmittancearound the peak of about 555 nm and with the steepest slope of theabsorption curve occurring between about 570 nm to 650 nm.

2. Description of the Prior Art

In order to obtain a green optical display filter that is compatiblewith night vision imaging systems, the filter must have a strong nearinfrared absorption to prevent the display lighting from interferingwith the operation of night vision goggles which is quite sensitive tonear infrared radiation. This type of display filter is required to havea transmittance of less than 10⁻⁶ for near infrared radiation of 700 nmto 950 nm and less than 5×10⁻⁶ at 650 nm to 700 nm. At the same time, itshould have a transmission peak for green radiation of 550 nm to 565 nm.

One conventional approach to achieving these requirements involvescostly and laborious arrangements of cementing a polished infraredsuppressive glass filter onto another polished color correction glassfilter. However these two glasses must be selected so as to have similarif not identical thermal expansion properties in order for the cement tocontinue to hold the glasses together through temperature and weathervariations. The additional color correction layer will also causefurther inconvenience when the display demands a thin window filter.

The use of alkali metal or alkaline earth metal phosphate glass used ashost glass for colored optical filters has also been proposed whereinthe phosphate glasses contain CuO as a coloring agent, e.g. see U.S.Pat. Nos. 4,303,298, 4,110,245 and 4,615,989 in this regard.

SUMMARY OF THE INVENTION

Accordingly an object of the present invention is to provide filterphosphate-based glass suitable for use with night vision image systemdisplays having a high near infrared absorption at about 650 nm to 950nm and a transmission peak at 550 nm to 565 nm. The narrow wavelengthdifference between the transmission peak and beginning of near infraredabsorption at 650 nm also indicates a requirement of a steep slope ofthe absorption curve occurring between about 570 nm and 650 nm.

Another object of the present invention is to use Cr₂ O₃ and CuO asco-dopant coloring agents in the glass in order to attain the chromaticand optical properties sought.

It has been discovered that the above object and requirements can befulfilled with glasses having the following compositions (in mol % ofthe glass composition):

    ______________________________________                                        50-60% P.sub.2 O.sub.5,                                                                        0-5%     SiO.sub.2,                                                                           20-33% Li.sub.2 O,                            0-25% Na.sub.2 O,                                                                              1-25%   K.sub.2 O,                                                                           0-5%   R.sub.2 O,                             7-10% Al.sub.2 O.sub.3,                                                                       3-6%     CuO,   0.5-1.6%                                                                             Cr.sub.2 O.sub.3,                     0.5-2.0%                                                                             HO.sub.2 O.sub.3,                                                                       0.5-2.0% Dy.sub.2 O.sub.3,                                                                      0-2.0%                                                                             Er.sub.2 O.sub.3,                     ______________________________________                                    

R₂ O represents any of MgO, CaO, SrO and BaO.

A preferred glass composition for obtaining the above object andrequirements is as follows (in mol % of the glass composition):

    ______________________________________                                        57.0% P.sub.2 O.sub.5,                                                                      7.7%   Al.sub.2 O.sub.3,                                                                   24.9% Li.sub.2 O,                                                                         1.6% K.sub.2 O                         5.1%  CuO,    1.6%   Cr.sub.2 O.sub.3,                                                                   1.0%  Ho.sub.2 O.sub.3                                                                    1.1% Dy.sub.2 O.sub.3                  ______________________________________                                    

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a graph showing the transmission curve of a 2 mm thickglass sample of the following glass composition (in mol %).

    ______________________________________                                        57.0% P.sub.2 O.sub.5,                                                                      7.7%   Al.sub.2 O.sub.3,                                                                   24.9% Li.sub.2 O,                                                                         1.6% K.sub.2 O                         5.1%  CuO,    1.6%   Cr.sub.2 O.sub.3,                                                                   1.0%  Ho.sub.2 O.sub.3,                                                                   1.1% Dy.sub.2 O.sub.3                  ______________________________________                                    

DETAILED DESCRIPTION OF THE INVENTION

As a result of extensive investigations on the relationships between thetype and content of various glass additives and their respectiveinfluence on the optical, chromatic as well as rheological properties ofthe glass material, the present inventors have discovered a novel glasscomposition, described in greater detail hereinafter, which possessesthe aforementioned absorption and transmittance characteristics sought.

In the compositional investigations each experimental melt was made with90 to 100 grams of glass. The thoroughly mixed batch material was meltedin a platinum crucible of 120 ml capacity for about 90 minutes followedby a few minutes mechanical stirring and a refining period of about 20minutes. Melting temperature ranged from 1140° to 1200° C. Highertemperatures of 1180° C. to 1200° C. were required for melting glasseswith higher Al₂ O₃ or SiO₂ content.

Evaluation of optical absorption for the test melt glasses was made bydetermining the optical absorption coefficient of the glass in thespectral range of 500 nm to 900 nm. Since most of the experimentalglasses have very high near infrared absorption, samples prepared forthis evaluation were made with thickness of 0.5 mm or less.

Variation of the type or content thereof of any component of theaforesaid composition can influence glass character. The influentialeffect and any content limitations of each component have beeninvestigated and are summarized hereinafter.

A higher concentration of P₂ O₅ causes larger amounts of weight lossduring the melting process because of the high volatility of P₂ O₅.Further, P₂ O₅ causes higher viscosity and lower stability againstdevitrification of the melt. When the content of P₂ O₅ exceeds 60 mol %,it becomes very difficult to obtain good quality glass with sufficientcoloring dopants included in the composition.

On the other hand, lowering of the P₂ O₅ concentration tends to reduceglass transition temperature which also decreases glass stabilityagainst devitrification. Another detrimental effect of lowering P₂ O₅concentration is the increase of the linear thermal expansioncoefficient of the glass.

These effects are demonstrated by the following results, wherein Tgrepresents glass transition temperature and α represents the linearthermal expansion coefficient (200°-300° C.).

    ______________________________________                                        Composition (in mol %)   α                                                    Dopants +                  (200-                                              other                 Tg   300° C.)                                                                       Glass                                Al.sub.2 O.sub.3                                                                    oxides*   Li.sub.2 O                                                                           P.sub.2 O.sub.5                                                                    (°C.)                                                                       (10.sup.-7 /°C.)                                                               Quality                              ______________________________________                                        8.2   10.4      37.0   44.4 401  130     particle                                                                      inclusions                           8.2   10.4      32.1   49.3 411  123     good                                 8.2   10.4      27.2   54.2 420  115     good                                 7.7   10.4      24.9   57.0 423  112     good                                 8.2   10.4      22.3   59.1 431  107     particle                                                                      inclusions                           8.2   10.4      17.4   64.0              devitrified                          ______________________________________                                         *Other oxides and dopants includes 1.6% K.sub.2 O, 5.1% CuO 1.6% Cr.sub.2     O.sub.3, 1% Ho.sub.2 O.sub.3 and 1.1% Dy.sub.2 O.sub.3.                  

Based on these observed effects of P₂ O₅ concentration, the preferredconcentration for this component has been determined to be 50-60 mol %for this invention.

The amount of SiO₂ allowed in this present glass composition is lessthan 5 mol %. Higher silica content is beneficial for the glass insofaras lowering its linear thermal expansion coefficient and improvingchemical resistance. However, the high content of SiO₂ tends to causesome coloring dopant crystal formation in this glass. Results tabulatedhereinafter show these effects.

    ______________________________________                                        Composition (in mol %) α                                                Al.sub.2 O.sub.3 +        Tg   (200-300° C.)                           Dopants*                                                                             Li.sub.2 O                                                                           P.sub.2 O.sub.5                                                                      SiO.sub.2                                                                          (°C.)                                                                       (10.sup.-7 /°C.)                                                                Glass Quality                         ______________________________________                                        18.6   32.1   49.3   0.0  411  123      Good                                  18.6   27.2   49.3   4.9  420  114      Good                                  18.6   22.3   54.2   4.9  430  107      Good                                  18.6   17.4   59.1   4.9  437  98       Particle                                                                      Inclusions                            18.6   22.3   49.3   9.8  438  98       Particle                                                                      Inclusions                            18.6   17.4   54.2   9.8  458  99       Particle                                                                      Inclusions                            18.6   17.4   49.3   14.7 452  99       Particle                                                                      Inclusions                            ______________________________________                                         *Al.sub.2 O.sub.3 plus dopants include 8.2% Al.sub.2 O.sub.3, 1.6% K.sub.     O, 5.1% CuO, 1.6% Cr.sub.2 O.sub.3, 1% Ho.sub.2 O.sub.3, 1% Dy.sub.2          O.sub.3.                                                                 

Another effect of SiO₂ content in this glass is the reduction of opticalabsorption between 550 nm-900 nm. The magnitude of reduction is verysmall as SiO₂ concentration is varied in the range of 0-5 mol %. Theevaluated optical absorption coefficient reduction is very close to ourinstrumental error limits and tolerance.

Therefore, for purposes of this present invention, the optimalconcentration of SiO₂ is determined to be in the range of 0-5 mol %.

The influential effect and content limitation of Li₂ O in the presentglass have been investigated in a range of 17.4 mol % to 37.0 mol %. Allof the test melt glasses with 17.4 mol % of Li₂ O have been found tocontain particle inclusions. There is a general indication that higherLi₂ O content causes a better dissolution of coloring dopants. But thereare some detrimental effects for high Li₂ O concentration in this glass.It reduces both chemical resistance and thermal stability againstdevitrification. Glasses melted with 37.0 mol % of this component havebeen found to have high inclusion density and slight devitrification.Li₂ O also increases the linear thermal expansion coefficient of thisglass. Results in the following table show this trend.

    ______________________________________                                                                 α                                              Composition (in mol %)   (200-300° C.)                                 Dopants*                                                                              Al.sub.2 O.sub.3                                                                      P.sub.2 O.sub.5                                                                       SiO.sub.2                                                                           Li.sub.2 O,                                                                          (10.sup.-7 /°C.)                  ______________________________________                                        10.4    8.2     54.2    4.9   22.3   107                                      10.4    8.2     49.3    4.9   27.2   112                                      10.4    7.7     57.0    0     24.9   114                                      10.4    8.2     54.2    0     27.2   115                                      10.4    8.2     49.3    0     32.1   123                                      ______________________________________                                         *Dopants include 1.6% K.sub.2 O, 5.1% CuO, 1.6% Cr.sub.2 O.sub.3, 1%          Ho.sub.2 O.sub.3, and 1% Dy.sub.2 O.sub.3.                               

Another general trend that is associated with increasing Li₂ O contentis related to optical absorption of this glass. Increase of Li₂ Ocontent improves optical absorption of the glass. Absorptioncoefficients of several selected wavelengths for certain test meltglasses are tabulated below to demonstrate this trend.

    ______________________________________                                                  Composition (mol %)**                                               Glass No.   Al.sub.2 O.sub.3                                                                      SiO.sub.2  P.sub.2 O.sub.5                                                                    Li.sub.2 O                                ______________________________________                                        A           8.2     0          49.3 32.1                                      B           8.2     0          54.2 27.2                                      C           7.7     0          57.0 24.9                                      D           8.2     0          49.3 32.1                                      E           8.2     4.9        49.3 27.2                                      F           8.2     4.9        54.2 22.3                                      ______________________________________                                         **Other components include 1.6% K.sub.2 O, 5.1% CuO, 1.6% Cr.sub.2            O.sub.3, 1% Ho.sub.2 O.sub.3, 1% Dy.sub.2 O.sub.3.                       

Absorption Coefficient (1/cm) at the following wavelengths:

    ______________________________________                                                        575          650        750                                   Glass No.                                                                             554 nm  nm     600 nm                                                                              nm   700 nm                                                                              nm   800 nm                           ______________________________________                                        A       9.07    15.2   30.9  69   85    87   93                               B       8.21    12.4   25.7  62   77    76   81                               C       7.49    11.7   24.5  61   73    72   79                               D       9.07    15.2   30.9  69   85    87   93                               E       8.96    13.9   27.5  67   83    83   89                               F       8.11    12.0   24.9  61   77    75   79                               ______________________________________                                    

The increase of optical absorption due to higher Li₂ O concentrationinvolves the whole spectrum of 550 nm to 900 nm. Since the purpose ofthis invention is to develop a glass with strong near infraredabsorption and good transmission for 550 nm to 560 nm radiation, theoverall increase of optical absorption is not necessarily a benefit.However, the Li₂ O concentration does provide a useful parameter foroptical absorption adjustment.

The appropriate concentration of Li₂ O component in the glass of thepresent invention is determined to be 20 to 33 mol %.

Replacement of Li₂ O by either Na₂ O or K₂ O affects a change in theCu²⁺ ion's absorption behavior, especially for the radiation spectralregion of 550 nm to 700 nm. The transmission curve of this region tendsto be shifted to the longer wavelength. This red shift effect reducesoptical absorption between these two wavelengths. However the glassabsorption for the radiation of 750 nm to 900 nm peak area issubstantially increased. Therefore, the replacement of Li₂ O by Na₂ O orK₂ O might still be applicable and is within the scope of the presentinvention.

The total amount of alkali metal oxides preferable for the glasscompositions of the present invention is found to be less than 33 mol %with the Li₂ O mol % being greater than the sum of other alkali metaloxides.

Application of alkaline earth oxides in this glass has been found toreduce the solubility of coloring dopants. When more than 5 mol % forany one of MgO, CaO, or SrO was included, particle inclusions wereobserved.

The mol % concentration of Al₂ O₃ in this present glass is preferred tobe in the range of 7.0 to 9.0. Higher content of this component tends toreduce thermal expansion coefficient. However, when the Al₂ O₃concentration exceeds 9 mol %, the high melt viscosity and high liquidustemperature of the glass make casting more difficult. Glass melted with10 mol % of Al₂ O₃ and sufficient coloring dopants has been found tohave some particle inclusion inside. The following results show thistrend.

    ______________________________________                                                       α                                                        Composition** (mol %)                                                                        (200-300° C.)                                           P.sub.2 O.sub.5                                                                     Li.sub.2 O                                                                            Al.sub.2 O.sub.3                                                                       (10.sup.-7 /°C.)                                                                Glass Quality                                 ______________________________________                                        57.5  25.2    6.6      117      Good                                          57.0  24.9    7.7      112      Good                                          56.2  24.5    8.8      113      Good                                          55.4  24.2    9.9      109      Particle Inclusions                           ______________________________________                                         **Other components include 1.6% K.sub.2 O, 5.1% CuO, 1.6% Cr.sub.2            O.sub.3, 1% Ho.sub.2 O.sub.3, 1% Dy.sub.2 O.sub.3.                       

Coloring dopants selected for the present invention include CuO and Cr₂O₃ used in combination as the primary coloring dopants, and Ho₂ O₃, Er₂O₃, and Dy₂ O as supplemental dopants. Cu⁺² ions in phosphate glass havea broad and strong absorption band ranged from about 500 nm to 2000 nmwith peak located at about 800 nm. Cr³⁺ ions have absorption peaks atabout 450 nm and 650 nm with transmission peak at about 550 nm. Cr₂ O₃creates the green coloration for this glass. Cr₂ O₃ gives an absorptionpeak at 650 nm which will improve the capability of optical absorptionbetween 650 nm and 900 nm. This compound also increases the steepness ofthe absorption curve between 570 nm and 700 nm.

Ho³⁺ and Dy³⁺ ions have small absorption bands at about 650 nm and 800nm respectively. Er³⁺ ions have absorption peaks at both 650 nm and 800nm. Therefore, the addition of these rare earth oxides can be useful.The absorption of Ho³⁺ at 650 nm is important for this present inventionbecause of its improvement to the sharpness of the absorption curve.

Molar extinction coefficients of these rare earth ions, i.e.. Ho³⁺,Er³⁺, Dy³⁺, for near infrared radiation wavelengths are substantiallysmaller than Cu²⁺ and Cr³⁺ ions. In this present invention. CuO and Cr₂O₃ are considered as primary coloring dopants. Ho₂ O₃, Dy₂ O₃ andoptionally Er₂ O₃ are considered as necessary secondary or supplementaldopants since dissolution limits exists for CuO and Cr₂ O₃ in thepractice of the present invention.

If CuO dopant is solely used as coloring agent in this glass, itsconcentration could be increased up to about 11 mol % without anydevitrification in the glass. However, when the multiplicity of coloringdopants mentioned in the previous paragraph are used in combination themaximum concentration for the CuO dopant is reduced to about 6 mol %.Further increase of the CuO concentration causes particle inclusions inthe glass.

The dissolution limit of Cr₂ O₃ in the glass of the present invention iseven more stringent than that for CuO. The maximum concentration limitfor Cr₂ O₃, as co-doped or even if used as a sole dopant, is found to beabout 1.6 mol %. When the content of this dopant was increased to 2 mol%. devitrification of the glass occurred.

The preferred concentration of the secondary coloring dopants of Ho₂ O₃and Dy₂ O₃ has been found to be about 1.0 mol % each. Ho⁺³ ions have aweak absorption band stretching from 530 nm to 553 nm. Should the hightransmission at 555 nm become very critical, reductions of Ho₂ O₃concentration might be necessary. Er³⁺ ions have overlapping absorptionbands stretched from about 500 nm to 560 nm which causes the applicationof Er₂ O₃ in this glass to become more restricted. Therefore, the Er₂ O₃dopant is suitable only when the required transmission peak is locatedat wavelengths of 560 nm or greater.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit or scope thereof.

What is claimed is:
 1. A phosphate-based green filter glass with strong absorption in the spectral range of 650 nm to 950 nm, consisting essentially of the following ingredients, in mol %:(i) 50 to 60% P₂ O₅ (ii) 0 to 5% SiO₂ (iii) 20 to 33% Li₂ O, 0 to 25% Na₂ O, 1 to 25% K₂ O (iv) 0 to 5% MgO, 0 to 5% CaO, 0 to 5% BaO, 0 to 5% SrO (v) 7 to 10% Al₂ O₃ (vi) 3 to 6% CuO (vii) 0.5 to 1.6% Cr₂ O₃ (viii) 0.5 to 2.0% Ho₂ O₃ (ix) 0.5 to 2.0% Dy₂ O₃ (x) 0 to 2.0% Er₂ O₃.
 2. The phosphate-based filter glass composition as in claim 1, wherein the total mol % of monovalent oxides is 20 to 33 mol %.
 3. The phosphate-based filter glass composition as in claim 1, wherein the ratio of Cr₂ O₃ mol % to CuO mol % is between 0.25 to 0.80.
 4. A green filter glass consisting essentially of the following ingredients, in mol %: 57.0% P₂ O₅, 24.9% Li₂ O, 1.6% K₂ O, 7.7% AL₂ O₃, 5.1% CuO, 1.6% Cr₂ O₃, 1.0% Ho₂ O₃, 1.1% Dy₂ O₃.
 5. A green filter glass consisting essentially of the following ingredients, in mol %: 54.2% P₂ O₅, 4.9% SiO₂, 22.3% Li₂ O, 1.6% K₂ O, 8.2% Al₂ O₃, 5.1% CuO, 1.6% Cr₂ O₃, 1% Ho₂ O₃, 1.1% Dy₂ O₃.
 6. A green filter glass consisting essentially of the following ingredients, in mol %: 54.2% P₂ O₅, 4.9% SiO₂, 22.3% Li₂ O, 1.6% K₂ O, 8.2% Al₂ O₃, 5.1% CuO, 1.6% Cr₂ O₃, 2.1% Ho₂ O₃.
 7. A phosphate-based green filter glass as in claim 1, wherein said glass has a transmission peak at 550 nm to 560 nm and with the steepest slope of the absorption curve occurring between 570 nm and 650 nm. 